Fast Response Capacitive Gauging System Featuring Steep Slope Filter Discrimination Circuit

ABSTRACT

The high speed, high accuracy capacitive gauging system employs an oscillator fed through steep slope filter that is discriminates between very small changes in capacitance even in the presence of electrical noise. During intervals when the probe tip is retracted, the oscillator frequency is calibrated to match the sweet spot in the center of the linear operative region of the steep slope filter. This calibrates the system to overcome the effects of varying temperature and humidity in the manufacturing environment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/244,742, filed on Sep. 22, 2009. The entire disclosure of the above application is incorporated herein by reference.

FIELD

The present disclosure relates generally to capacitive gauging systems and more particularly to an improved capacitive gauging system for use in applications such as industrial laser processing, including but not limited to cutting, scribing, ablating and welding, and for use in other applications where the position of a moving member is controlled or measured using capacitive gauging. The system described herein can provide a means of measuring the tip-to-part standoff distance, for example. This distance measurement may be used as feedback to a servo loop to maintain a constant tip-to-part standoff, for example.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art. In capacitive gauging systems there are applications, such as laser processing, where extremely fast response times, noise immunity and compensation of background capacitance are essential. The ability of a system to excel in these features would make a product more effective and superior to other designs.

For example, maintaining a constant standoff height is advantageous in a laser process, as it keeps the laser focus consistent in relation to the part, as well as keeping the gas flow dynamics consistent. Using capacitance to measure the tip-to-part standoff distance is the preferred method for laser processing; however there exists some significant design challenges. Among these challenges are:

-   -   a) Fast laser processing results in proportionally fast         tip-to-part standoff compensation. Typical response times for         capacitive gauging systems are slower than 2 msec, and thus         place a practical limit on laser processing speeds.     -   b) Industrial laser processing is inherently electrically noisy.         The plasma and ejected molten metal can interfere with the         capacitance thus disrupting the ability of the system to         accurately measure the tip-to-part standoff. The typical way of         dealing with this interference is to filter it out with a low         pass filter with a cut-off frequency of ˜500 Hz., again         resulting in a relatively slow response time that degrades         overall laser processing speed.     -   c) The capacitance variation seen between the tip and the part         can be less than a picofarad. For this reason, keeping the         background capacitance constant is essential for accurate         tip-to-part gauging. However, variations in temperature,         humidity and other external influences can alter the background         capacitance, causing significant errors in the tip-to-part         measurement. Conventional systems have difficulty dealing with         this error.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

The improved capacitive gauging system described more fully herein has a unique analog circuit that employs a carefully calibrated capacitance-responsive oscillator and a high-frequency, high-order, low pass filter to process the output of the oscillator and thus measure minute changes in capacitance even when those changes occur very rapidly. The improved gauging system has a potential response time of faster than 0.01 msec and is thus considerably faster to respond than conventional (2 msec) capacitive gauging systems.

Instead of using a conventional 500 Hz. low pass filter to filter out laser-induced noise (at the expense of speed), the improved capacitive gauging system uses its high-frequency, high-order, low pass filter to rapidly discriminate between minute changes in capacitance. Thus the improved capacitive gauging system concentrates on quick recovery from noise, instead of attempting to filter it out.

To address environmental variances (e.g., in temperature and/or humidity) the improved capacitive gauging system uses a compensation circuit to adjust for the background capacitance when the tip is a relatively far distance away from the part. The compensation circuit is engaged at intervals between the cutting, scribing, ablating and/or welding processes. It accurately calibrates the oscillator to match the “sweet spot” of the high-frequency, high-order, low pass filter, so that the filter can continue to discriminate very small changes in capacitance as the probe tip-to-part distance changes.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 is a block diagram of the improved capacitive gauging system shown in context with the servomotor system used to drive the up-down movement of the tip relative to the part;

FIG. 2 is a detailed block diagram of the improved capacitive gauging system, with superimposed graphs showing various relationships among distance (d), capacitance (C), frequency (freq), amplitude (amp) and voltage out (Vout);

FIG. 3 is a graph showing how filter slope affects discrimination ability of the capacitance sensor; and

FIG. 4 is an oscilloscope waveform diagram showing how the height (sense signal) produced by the capacitive sensor compares with the position indicator produced by the gauging system in the presence of noise.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference to the accompanying drawings.

With reference to FIG. 1, the improved capacitive gauging system is shown generally at 10. As illustrated, the gauging system supplies a control signal to the motor driver circuit 12, which in turn controls the up-down motion of the tip servomotor 14. For illustration purposes, FIG. 1 shows the tip 16 in two positions: position 16 a—in working proximity to the part 18 and position 16 b—in retracted position. It will be understood that during use, the servomotor 14 is controlled to maintain the proper working distance (d) between tip 16 and part 18. As will be more fully described below, from time to time, such as between intervals of working interaction between tip and part (e.g., during cutting, scribing, ablating and/or welding), the servomotor 14 retracts the tip to position 16 b. When retracted to position 16 b, the compensation calibration circuit 20 performs a calibration operation upon the capacitance sensor 22. Details of the compensation calibration circuit 20 and capacitance sensor 22 are provided below in connection with FIG. 2.

The improved capacitive gauging system employs a unique analog circuit that performs highly accurate and rapid capacitive sensing. A digital circuit, cycle control processor 24 mediates the operation of this analog circuit. The cycle control processor 24 sends digital control instructions to the motor driver 12 to cause the tip servomotor 14 to retract the tip to position 16 b. This retraction motion would typically be performed after a particular working interaction between tip and part have completed, such as when it is time to remove a finished part and insert a fresh part to be worked upon. Upon retraction of the tip, the cycle control processor 24 sends digital control instructions to the compensation calibration circuit 20, causing it to calibrate the capacitance sensor 22 for its next use.

Referring now to FIG. 2, the capacitance sensor 22 of the improved capacitive gauging system 10 employs an oscillator 26. The frequency generated by the oscillator is a function of resistance (R), capacitance (C) and/or inductance (L). The capacitance (C) portion of the function is itself a function of the distance (d) between the tip and the part. As the distance between the tip and the part changes, the capacitance changes, resulting in a change in the oscillator frequency. The resulting frequency is nominally between 1 KHz and 1 GHz. The oscillator can be based on a LM 555 timer circuit in “Astable operations” mode (see data sheet www.natural.com/ds/LM555.pdf) optional modification to this circuit can be used to reduce the circuit's susceptibility to temperature variations. For example, the circuit can be separated into an input buffer, comparator, and flip-flop.

The output of the oscillator 10 is sent through a high-frequency, high-order (steep slope) low-pass filter 28. The low pass filter 28 can be implemented as active filter with a cut-off frequency close to the operating frequency of the oscillator. In a presently preferred embodiment filter 28 is a 7^(th) order filter, which provides a very steep cut-off curve. In general, the steeper the cut-off curve, the more sensitive the resulting system will be. The resulting output of filter 28 will have an amplitude that increases as the frequency decreases. This can be illustrated with reference to FIG. 3, which shows the slope of an exemplary low-pass filter. Note that the filter is designed with its “sweet spot” 30 centered at a frequency that matches the nominal frequency of the oscillator 26 when the tip is in its retracted position 16 b. Because of the steep cut-off slope, a small change in oscillator frequency (x-axis) will produce a large change in filter output amplitude (y-axis). In order to achieve the high order (greater than 4) of filtering, commercially available active filters are implemented with appropriate support circuitry (e.g., datasheets.maxim-ic.com/en/ds/max280-mxl1062.pdf).

While a high-frequency, high-order low-pass filter is presently preferred, the capacitance sensor 22 may also be implemented using a high-pass filter or notch filter. Essentially, any filter that provides a steep slope that can produce a large change in filter output for a small change in oscillator frequency will work. If a high-pass filter is chosen, for example, the curve illustrated in FIG. 3 would be flipped so that linear operative region would slope in the opposite direction. While this would change the polarity of the filter output, essentially the same frequency discrimination can be had.

To measure the output amplitude of filter 28, the capacitance sensor 22 employs a radio-frequency detector 32. The radio frequency detector is tuned to receive frequencies covering the frequencies associated with the linear operative region of filter 28. The radio-frequency detector 32 thus produces an output signal that, in effect, converts the varying amplitude output of filter 28 into a varying voltage (V_(out)). The RF detector 32 produces a DC voltage that is proportional to the power in the signal that results from the bypass filter (low-pass) 28. The RF detector can be implemented using a diode based rectifier circuitry or dedicated integrate circuit (see datasheets.maxim-ic.com/en/ds/max2016.pdf). To improve signal response, a suitable gain stage 34 may be included, with its amplified output fed to a final output stage 36 which feeds and/or comprises part of the motor driver circuit 12 (FIG. 1). The motor output stage optimizes the signal and conditions it for use as a gauging system. This signal thus feeds the tip servomotor 14 which translates the V_(out) signal into a physical position (d). The gain stage can be a standard op-amp based differential filter.

To work effectively, the sweet spot 30 of the filter (FIG. 3) should be positioned at or near the nominal frequency of the oscillator 26. As explained above, the oscillator frequency varies in relation to the sensed capacitance. Capacitance, in turn, varies as the distance between tip and part changes. However, capacitance is also affected by environmental conditions, such as temperature and humidity, and these environmental factors can significantly degrade accuracy. To compensate for environmental variances, the improved capacitive gauging system employs a compensation calibration circuit 20, which utilizes a background capacitance compensator circuit 38. Compensator circuit 38, when engaged, forms a feedback loop between the output of gain stage 34 and the oscillator 26. The compensator circuit 38 is selectively connected (in response to instruction from the cycle control processor 24, FIG. 1) when the tip is in its retracted position 16 b. The background capacitance compensator circuit uses closed loop feedback control to accurately set the oscillator frequency of oscillator 26 by adjusting parameters of the oscillator frequency function to normalize the resulting signal when the tip is far from the part, or some other known position.

Unlike conventional capacitive gauging systems, which employ heavy low pass filtration (e.g. 500 Hz.) to filter out laser process-induced noise, the improved capacitive gauging system is able to operate in the presence of noise. To illustrate, refer to FIG. 4, which shows the output of a dual trace oscilloscope. The uppermost waveform depicts the output of the capacitive sense signal and the lowermost waveform depicts the position indicator output of the gauging system. Note that the capacitive sense signal has a lot of noise that is created by the laser process and is detected by the height sensing circuit. Even in the presence of this noise, the resulting motion shown on the bottommost waveform shows negligible noise.

From the foregoing it will be seen that the improved capacitive gauging system advantageously utilizes a blend of analog and digital circuitry to produce fast and highly accurate measurements even in the presence of noise. The system uses the tip retraction intervals to perform environmental calibration, thus ensuring high accuracy even in the face of temperature and humidity variations.

While the improved capacitive gauging system has been illustrated and described in its presently preferred form, it will be understood that certain modifications can be made to the circuitry or selected components without departing from the spirit of our inventive concept.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

1. A capacitive sensing system for measuring the height of a laser head above a work piece, said system comprising: a laser head nozzle forming a first plate of a capacitor; the work piece forming a second plate of the capacitor; a capacitive-responsive radio frequency oscillator operably coupled to the capacitor, said oscillator providing an output; and a high-frequency, high-order bypass filter to process the output from the oscillator to measure minute changes in capacitance of the capacitor.
 2. The capacitive sensing system of claim 1, further comprising an actuator configured to move the laser head nozzle in response to the minute changes in capacitance of the capacitor.
 3. The capacitive sensing system according to claim 1, wherein the high-order bypass filter has a response time of faster than about 0.01 msec.
 4. The capacitive sensing system according to claim 1, further comprising a temperature/humidity compensation circuit configured to compensate for changes in background capacitance.
 5. The capacitive sensing system according to claim 4, further comprising a controller configured to engage the compensative circuit at intervals between successive activation of the laser nozzle.
 6. The capacitive sensing system according to claim 5, wherein the successive activation of the nozzle is associated with one of cutting the work piece, scribing the work piece, ablating the work piece, welding the work piece, and combinations thereof.
 7. The capacitive sensing system according to claim 1, wherein the bypass filter is one of a low-pass filter, a high-pass filter, and a notch-filter.
 8. The capacitive sensing system according to claim 1, further comprising a radio-frequency detector coupled to the high frequency high-order bypass filter.
 9. The capacitive sensing system according to claim 8, wherein the radio frequency detector converts a varying output of the filter into a varying voltage.
 10. A sensing system for a laser, the system comprising: a laser having a nozzle; an actuation mechanism coupled to the nozzle; a capacitive sensing system comprising a capacitor formed of the nozzle and a work piece; a capacitive response oscillator operably coupled to the capacitive sensing system, said capacitive response oscillator varying from a first radio frequency to a second radio frequency in response to changes in capacitance of the capacitive sensing system; a high-order bypass filter to process an output signal from the capacitive response oscillator, said high-order bypass filter being a greater than 4^(th) order bypass filter; and a radio frequency detector configured to convert a varying output of the filter into a varying voltage, wherein said actuator mechanism is operably coupled to the radio frequency detector to move the nozzle in response to the varying voltage.
 11. The sensing system according to claim 10, further comprising a capacitive compensation circuit operably coupled to the bypass filter.
 12. The sensing system according to claim 11, wherein the compensation circuit forms a feedback loop between an input of a gain stage and the oscillator.
 13. The sensing system according to claim 12, wherein the compensation circuit is selectively engaged when the nozzle is in an unengaged condition.
 14. The sensing system according to claim 10, wherein the bypass filter is a high-order low-pass filter.
 15. A capacitive gauging system for a laser, said system comprising: a capacitor formed from a structural portion of the laser and a work piece; a radio frequency oscillator configured to detect changes in capacitance of the capacitor; a high-frequency high-order bypass filter coupled to the oscillator, said bypass filter being a greater than 4^(th) order filter to measure minute changes in an output of the radio frequency oscillator; and a gain stage coupled to the radio frequency detector, said gain stage configured to drive a laser head drive mechanism.
 16. The capacitive gauge system according to claim 15, further comprising a background capacitance compensator configured to adjust the output of the oscillator.
 17. The capacitive gauge according to claim 15, wherein the bypass filter is a low-pass filter.
 18. The capacitive gauge according to claim 15, wherein the drive mechanism comprises a servo-motor.
 19. The capacitive gauge according to claim 18, further comprising a cycle control processor coupled to the drive motor.
 20. The capacitive gauge according to claim 15, wherein the radio frequency oscillator comprises an input buffer, a comparator, and a flip-flop. 